The present invention generally relates to the continuously variable transmission art. More particularly, the present invention relates to clutch control for continuously variable transmissions and to providing desired clutch control in particular driving circumstances.
The art contains numerous examples of the operation and construction of continuously variable transmissions (CVT's), e.g., U.S. Pat. No. 4,522,086 and U.S. Pat. No. 4,458,318, entitled Control System for Continuously Variable Transmission, and Control Arrangement for a Variable Pulley Transmission, respectively. These patents generally describe the mechanics and controls for a CVT system utilizing two adjustable pulleys, each pulley having at least one sheave which is axially fixed and another sheave which is axially movable relative to the first sheave. A flexible belt of metal or elastomeric material intercouples the pulleys. The interior faces of the pulley sheaves are beveled or chamfered. Thus, as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. The displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter; as fluid is exhausted from the chamber, the pulley diameter changes in the opposite sense. Generally, the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby effecting a change in the drive ratio between the input shaft coupled to an input pulley and an output shaft coupled to an output pulley. The ratio changes continuously as the pulley diameters vary. Such transmissions frequently are referred to in the art as a continuously variable transmission, CVT.
Through the years various developments have refined and improved the hydraulic control system which is used to pass fluid into the fluid holding chamber of each adjustable pulley. An example of such a hydraulic system is shown in U.S. Pat. 3,115,049--Moan. In that patent, control of the secondary pulley adjustable sheave regulates the belt tension, while a different circuit regulates fluid into and out of the primary sheave to regulate the transmission ratio. U.S. Pat. No. 4,152,947--van Deursen et al.-also describes control of a CVT. In both systems, the line pressure of the fluid applied to hold the belt tension by pressurizing the secondary chamber is kept at a relatively high value. An improved control system subsequently was developed to reduce the main line fluid pressures supplied to the secondary sheave chamber as a function of torque demand. This improved system is described and claimed in U.S. Pat. No. 4,522,086 entitled "Control System for Continuously Variable Transmission," which issued Jun. 11, 1985, and is assigned to the assignee of this application. Further work resulted in an improved control system which reduced the line pressure applied to the secondary chamber to a lower, safer operating pressure and also provided a lower control pressure for other portions of a hydraulic control system. This system is described in an application entitled "Hydraulic Control System for a Continuously Variable Transmission", Ser. No. 421,198, filed Sep. 22, 1982, which issued Dec. 15, 1987, as U.S. Pat. No. 4,712,453, and assigned to the assignee of this application.
Additional significant advances in CVT control systems have been described and claimed in another application also entitled "Hydraulic Control System for a Continuously Variable Transmission," Ser. No. 717,913, filed Mar. 29, 1985, which issued Jan. 12, 1988, as U.S. Pat. No. 4,718,308, and assigned to the assignee of this application.
Another U.S. Pat. No. 4,648,496, issued Mar. 10, 1987, entitled "Clutch Control System for a Continuously Variable Transmission" further describes control logic techniques for regulating pressure at a clutch in a CVT system to provide the desired torque transfer from an engine to a vehicle drive line. In that system, clutch control depends on logical recognition of one of a number of operating modes.
A related, co-pending application entitled "Continuously Variable Transmission Clutch Control System", Ser. No. 25,391, filed Mar. 13, 1987 assigned to the assignee of the present application, discloses an improved clutch controller for use in CVT applications. Another related, co-pending application entitled "Ratio Control Technique For Continuously Variable Transmission Systems", Ser. No. 25,389, filed Mar. 13, 1987 assigned to the assignee of the present application, discloses an improved belt ratio control technique for use in CVT applications.
It has been recognized that particular driving conditions may exist during which a normal start mode for operation of the clutch control may not be appropriate. For example, in U.S. Pat. No. 4,648,496, a special start mode determination circuit 126 determines that special conditions, other than those of normal start, exist as the driver moves the manual selector into drive and steps into the accelerator. However, that system does not specify a detailed technique for operation of the clutch control system during special start mode operation other than in general sense. Likewise, the co-pending application entitled "Continuously Variable Transmission Clutch Control System" merely references other conditions in which the normal start mode operation described therein would not be applicable.
The teachings of each of the above referenced patents and co-pending applications are incorporated herein by reference, although none of the previously envisioned systems specifically addresses implementation of the control techniques disclosed herein.